Photoaffinity labeling of human serum albumin, thyroxine-binding globulin, prealbumin or rat liver nuclear proteins (NE) or whole nuclei (N) with L-thyroxine or 3,5,3'-triiodo-L-thyronine was achieved by irradiation in the near U.V. The major nuclear proteins that were covalently labeled have Mr of about 56,000, 45,000 and 12,000, respectively. Similar labeling patterns were observed with 3,3',5'-triiodo-L-thyronine (rT3) and with N-bromo-acetyl-T3 (or -T4) (without irradiation).